a) Field of the invention
The present invention relates to a view-finder optical system to be used with photographic cameras, video cameras and so on, and more specifically to a real image type variable magnification view-finder optical system which is suited for panoramic photographing.
b) Description of the prior art
In the recent days, people are fond of photographs taken on films having slenderness ratios which are obtained by trimming the conventional film sizes (for example, 36 mm by 22 mm). As an example of these photographs taken on the films having such slenderness ratios, there is known a panoramic photograph which is obtainable by photographing an object while covering an upper portion and a lower portion of a film and enlarging a photographed image of the object at a printing stage thereof so as to elongate a photographed range of the object sideways for emphasizing a wide impression of the photographed image.
Since a field angle of incidence for trimming a film surface is smaller than a field angle of incidence for photographing an object on a full size of the film surface, it was conventionally necessary to narrow a picture plane of a view-finder of a camera having a panoramic photographing function in conjunction with narrowing of the field angle of incidence so as to maintain a constant field ratio for the panoramic photographing mode. Accordingly, a range of the visual field within which an object is to be photographed was indicated by using straight lines traced within a frame of the visual field or the picture plane of the view-finder was narrowed by shortening a vertical size of the frame of the visual field.
When the range of the visual field within which the object is to be photographed was indicated within the frame of the visual field, however, a photographer could not judge, so long as he was peeping through the view-finder, whether the camera is set in either of the photographing mode and sometimes misunderstood a range within which the object is to be photographed since the picture plane of the view-finder has a size remaining unchanged between a usual photographing mode and the panoramic photographing mode. In this case, the view-finder optical system has another defect that it permitted the photographer observing portions of the visual field which were not to be photographed, thereby hindering the trimming operation to be performed by the photographer. In the other case where the picture plane of the view-finder is narrowed by shortening the vertical size of the frame of the visual field, in contrast, an image observed through the view-finder in the panoramic photographing mode gave a contracted impression, contrary to the wide impression, whereby a photographed image often gave an impression which was often different from that of the image observed at a photographing stage.
In order to correct the defects described above, there have hitherto been contrived methods to enlarge sizes of picture planes of visual fields by enhancing magnifications of view-finders in the panoramic photographing mode. These methods make it possible to enlarge the picture planes in conjunction with field angles of incidence of photographic lens systems so that images observed through the view-finders gave wide impressions.
As one of the methods for enhancing the magnifications of the view-finders, there is known a method to preliminarily design zooming cams so as to permit obtaining vari-focal ratios which are higher than those required for the usual photographing mode and perform, at photographing stages, changes of magnifications for the panoramic photographing mode after shifting vari-focal lens units.
This method requires using two mechanisms for each view-finder: one mechanism required for shifting a vari-focal lens unit regardless of the photographing modes; and the other mechanism required for shifting lens units only in the panoramic photographing mode. As a result, the vari-focal lens unit must be shifted along a complicated locus and an assembly of these two mechanisms has a complicated structure, thereby posing a problem that the mechanisms can hardly be manufactured in practice.